The human herpes virus 8 has been detected in all forms of Kaposi's sarcoma, in primary effusion lymphomas (PEL), in Castleman's disease, in angiosarcomas, in skin lesions of patients who underwent transplantations, in plasmacytomas, sarcoidosis as well as in healthy control individuals (Chang et al., 1994; Boshoff and Weiss, 1997). Seroepidemiological studies have shown that in northern and central Europe HHV-8 is substantially restricted to risk groups and that there are large differences with respect to geography and age. Moreover, these studies were able to show that in patients with Kaposi's sarcomas a seroconversion for HHV-8 is detectable months to years prior to the diagnosis of Kaposi's sarcoma and that it is likely that HHV-8 is transferred primarily via sexual contacts. To date, the clinical signs of a primary HHV-8 infection are unknown. Due to the detection of HHV-8 in almost 100% of all Kaposi's sarcomas, the correlation of the regional seroprevalence with the incidence of HHV-8 and the seroconversion prior to the appearance of Kaposi's sarcoma it is believed today that HHV-8 at least represents a co-factor in the tumorigenesis of KS. Up to now, the role of HHV-8 in the other diseases mentioned above is unclear and the question whether HHV-8 is involved in other so far unidentified diseases is still unanswered.
Taxonomically, on the basis of sequence homologies HHV-8 belongs to the gamma herpes virus sub-family and is closely related to EBV and Herpesvirus saimiri. The HHV-8 genome is 140 kb in size and is flanked by several repetitive sequences having a length of approximately 800 bp (Russo et al., 1996). HHV-8 codes for about 80 proteins, 10 of which show homology to cellular gene products (Neipel et al., 1997). Similar to all other herpes viruses, HHV-8 is able to cause a lytic infection which then becomes a latent infection. In the latent phase, at least two viral transcripts are expressed: a differentially spliced mRNA encoding the v-cyclin, v-flip and LANA proteins, as well as T0.7, a short RNA 0.7 kb in length and of up to now unknown function (Zhong et al., 1996). The viral transcript T0.7 is the most abundant of the RNAs expressed in the latent phase and has three open reading frames corresponding to 60, 35, and 47 amino acids.
So far, a HHV-8 infection has been detected by polymerase chain reaction using HHV-8-specific oligonucleotide primers. This direct method of detection has disadvantages in that it (i) requires at lot of effort and costs (about five to ten times the price of serological detection), (ii) is susceptible to false positive results due to contamination, (iii) detects only acute infections but not earlier ones, and (iv) detects only 50% of all (acutely) infected subjects if used on peripheral blood.
The serological detection methods developed so far are based on either the use of HHV-8-infected cell lines or on recombinant viral proteins. Assays detecting the antibodies directed against HHV-8 on HHV-8-positive cell lines by means of immunofluorescence have the disadvantages that (i) their reproducibility is low (i.a. because it is impossible to keep the culture conditions for the HHV-8 cell lines absolutely constant), (ii) their evaluation may not be performed by machine which makes them unsuitable for a larger number of tests, and (iii) it is in part difficult to exclude cross-reactions of antibodies against other viruses. A common problem of all of the assays based on recombinant viral proteins ist their low sensitivity. This problem is based on the fact that antibodies are produced only against particular proteins of the virus and that different individuals may in part produce antibodies against different proteins. The viral proteins tested to date for their utility in serological diagnostics have a sensitivity of only between 30 and 80%. FIG. 1 exemplarily shows that antibodies against the minor capsid protein VP23 can be detected in only 30% of KS patients (FIG. 1). It is highly likely that the use of more than one viral protein is required in order to develop more sensitive assays.
Since with respect to its frequency Kaposi's sarcoma is in third place of the tumors occuring after an organ transplantation and according to current knowledge its occurence is closely associated to HHV-8 infection it may be expected that in the future organ donors and possibly also blood products will be tested for HHV-8 similar to e.g. the obligatory testing for HIV, hepatitis B and C which is performed today.
In part, the dignity of KS is highly variable. The KS which does not occur endemically is almost exclusively restricted to the immunodeficient patient and generally has a malignant course. It is treated by chemotherapy (such as liposomal doxorubicin), surgically or by means of radiotherapy with modest success. Retrospective studies indicate that the virostatics Foscarnet and Gancyclovir are effective. Larger prospective studies with regard to the effectivity of virostatics on KS have not been published to date. Also missing up to now is a uniform treatment schedule for the B cell lymphomas in which HHV-8 has been detected. Chemotherapeutic schedules have been widely used which were generally employed in the treatment of Non-Hodgekin lymphomas.